1. Field of the Invention
The present invention relates to a coating and method useful for the interferometric measurement of plano and spherical wavefront producing optical surfaces and systems. More particularly, the invention relates to the application of a partially absorbtive metallic or semi-metallic optical coating which is applied to the reference surface of a spherical or plano wavefront Fizeau interferometer to produce high contrast two-beam interference fringes for any test surface or system reflectivity.
2. The Prior Art
The development of the laser and advances in vacuum coating technology have greatly expanded the utility of classical interferometers. The Fizeau interferometer, in particular, has become an extremely convenient and flexible instrument for a wide variety of optical metrology applications. Nevertheless, a conspicuous shortcoming of the laser Fizeau interferometer has been in testing high reflectivity optical surfaces and systems which produce plano and spherical wavefronts. With the recent developments in phase measurement interferometry where high contrast, two-beam interference fringes are required to simplify the data analysis, this is especially germane, see M. Schaham, Proceedings SPIE, Vol. 306, pp. 183-191 (1981). A multiple-beam spherical wavefront Fizeau interferometer is discussed in detail by Heintze et al. in Applied Optics, Vol. 6, p. 1924 (November, 1967). The major difficulties with the interferometer discussed by Heintze et al. are: (1) the partially transmissive coating on the spherical reference surface must be selected to match closely the reflectivity of the test surface or system to achieve useful contrast. Therefore, a number of these expensive surfaces is required to handle a range of test surface or system reflectivity, and (2) a field lens which matches each test surface or system is required, and (3) multiple-beam interference fringes are produced, thereby excluding fringe analysis using phase measurement interferometry. A multiple-beam plano wavefront Fizeau interferometer has similar difficulties as mentioned for the multiple-beam spherical wavefront Fizeau interferometer with the exception that a field lens is not required. Another method is to use the Fizeau interferometer together with a thin coated pellicle placed into the interferometer cavity, i.e., the space between the reference surface and the test surface or system, having a transmission determined by the test surface or system reflectivity, see Hunter and Forman, U.S. Pat. No. 3,998,553 issued Dec. 21, 1976. The difficulties with this method are: (1) locating the pellicle in the interferometer cavity limits the proximity to which a test surface or system can be positioned relative to the reference surface. This severely limits the range of test surface or system surface curvatures that can be tested, and (2) the pellicle itself is expensive, delicate, and easily damaged, and (3) the introduction of the pellicle into the interferometer cavity in the presence of a strongly convergent or divergent measurement beam has the effect of introducing significant wavefront errors which seriously degrade the measurement accuracy of the interferometer, and (4) usually more than one pellicle is required for the full range of test surface or system reflectivity, and (5) the back reflections from the pellicle surface can produce spurious fringes or cause severe reductions in fringe contrast requiring the pellicle to be tilted with respect to the interferometer optical axis limiting still further the volume between the reference surface and the test surface or system.
Other types of interferometry are used to test high reflectivity optical surfaces and systems. For example, scatter plate interferometers and shearing-type interferometers are two prominent prior-art techniques. However, these interferometers are not only difficult to use and align, but they are also considerably less versatile than the Fizeau interferometer. The Twyman-Green two-beam interferometer, while being able to measure the full range of optical surface and system reflectivity without any additional surfaces in the interferometer cavity or any specialized coating on the reference surface, other than one to match the test surface or system reflectivity, has the disadvantage of being more complex and expensive. That is, following the beamsplitting surface, any optical surface or element in either arm of the interferometer up to and including the reference surface must be of high optical quality, whereas in the Fizeau interferometer, only the reference surface must be of high optical quality. Also, the interferometer cavity in a Twyman-Green interferometer is inherently longer than a Fizeau interferometer cavity and thus more susceptible to environmental noise.
While these prior-art techniques are useful for some applications, they cannot be used in those optical metrology applications requiring both phase measurement interferometry and a small separation between the reference surface and test surface or system. To this end, a coating and method are required for testing the full reflectivity range of expected optical surfaces and systems without the limitations of the above-mentioned prior-art.